It does depend on your childcare really - and age of child. When I only had one DC and she was little, I preferred to have her at her childminder for two or three full days and work that pattern. I have been lucky as she has got older (and DC2 has also arrived) I have been able to adjust my work pattern around them - so now I do 2 long days and 2 short days (one school hours and one just a morning).
I would say look at childcare options first then work around that.
I am doing some research for work as I will be going on Maternity leave shortly and we are looking into cover whilst I am away. My boss was wondering if anyone had any thoughts on what would be the ideal hours for a mum going back to work part time, but as he has never had this situation (and I am a first time mum), I was hoping that you may be able to offer some suggestions. Many thanks for your help.